The present invention provides compositions that can be used for a variety of applications including, for example, removing unwanted resist films, post-etch, silicon etching, and post-ash residue on a semiconductor substrate. In particular, the present invention provides etching compositions that are particularly useful for anisotropic and isotropic silicon etch.
The background of the present invention will be described in connection with its use in etching applications involving the manufacture of integrated circuits. It should be understood, however, that the use of the present invention has wider applicability as described hereinafter.
In the manufacture of integrated circuits, it is sometimes necessary to etch openings or other geometries in a thin film deposited or grown on the surface of silicon, gallium arsenide, glass, or other substrate located on an in-process integrated circuit wafer. Present methods for etching such a film require that the film be exposed to a chemical etching agent to remove portions of the film. The particular etching agent used to remove the portions of the film depends upon the nature of the film. In the case of an oxide film, for example, the etching agent may be hydrofluoric acid. In the case of a polysilicon film, it will typically be hydrofluoric acid or a mixture of nitric acid and acetic acid.
In order to assure that only desired portions of the film are removed, a photolithography process is used, through which a pattern in a computer drafted photo mask is transferred to the surface of the film. The mask serves to identify the areas of the film which are to be selectively removed. This pattern is formed with a photoresist material, which is a light sensitive material spun onto the in-process integrated circuit wafer in a thin film and exposed to high intensity radiation projected through the photo mask. The exposed or unexposed photoresist material, depending on its composition, is typically dissolved with developers, leaving a pattern which allows etching to take place in the selected areas, while preventing etching in other areas. Positive-type resists, for example, have been extensively used as masking materials to delineate patterns on a substrate that, when etching occurs, will become vias, trenches, contact holes, etc.
Wet etching is a process in which chemical compositions are used to dissolve the areas of a silicon substrate that are not protected by a mask. There are two different types of etching: isotropic etching and anisotropic etching. Isotropic etching is considered non-directional etching meaning that it erodes the substrate equally in all directions. A key disadvantage of isotropic etching is that it may cause undercutting or etching under the mask itself. Another disadvantage of isotropic etching is the chemicals used in the process. Typical isotropic etchant chemicals may include hydrofluoric acid which presents problems in handling, safety and disposal. Lastly, isotropic etching is prone to high defect levels due to particulate contamination and has poor process control.
Anisotropic etching, unlike isotropic etching, provides a directional etching of the substrate surface. In this regard, undercutting under the mask is sharpened to well-defined corners because the typical etchant chemicals used in this process such as potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), and tetraethylammonium hydroxide (TEAH), which etch at different rates for different crystal orientations such as Si(110), Si (100), and Si (111) crystal plans. Very specific etch patterns can be implemented due to the use of directional dependent etchants. For example, conventional TMAH-based etchant chemicals typically provide a “V shaped” profile. Etch rates can be further controlled by etchant type, concentration and temperature. Like isotropic etching, there are also drawbacks to anisotropic etching. The anisotropic etching process is orientation dependent which can cause issues. In this regard, silicon wafers should be chosen with a certain Miller index orientation to control the etch pattern in the wafer. Like isotropic etching, the etchant chemicals also present problems in handling, safety, and disposal.
FIGS. 1A through 1C provide a prior art example of an anisotropic etching process using a conventional TMAH-based etchant chemical to form a sigma-shaped recess in a substrate. FIG. 1A provides a silicon substrate having a surface feature such as a gate wherein the crystal plane orientation of the surface of the substrate is (100). FIG. 1B shows a U shaped recess defined by points A, B, C, and D which is formed in the substrate by dry etching. Arrows C and D show the crystal plane orientations (111) and (110). FIG. 1C shows the results of a wet anisotropic etching process using a TMAH-based etchant chemical. However, in a wet anisotropic etching process, the etching rate of the (100) and (110) crystal orientations are faster than that of the (111) crystal plane orientation. This can cause the bottom of the U-shaped recess to be over-etched wherein the lower portions of the opposite sidewalls of the recess to intersect and form a cusp such as a V-shaped or curved peak instead of the desired flat bottom. FIG. 2 provides a cross-sectional view of the desired sigma-shaped (Σ) recess in the substrate that has a gate.
Therefore, there is a need in the art for an etching composition that is non-toxic and environmentally friendly for etching processes. There is a need in the art for etching compositions that provide a wider processing window. There is a need in the art for etching compositions that can selectively etch certain crystal planes or perform crystal orientation selective wet etching and provide a flat bottom. There is an additional need in the art to provide an etching composition that can provide an Σ-shaped recess for anisotropic etching. There is a further need in the art for an etching composition for use in the manufacture of semiconductors that provides an alternative to conventional etchant compounds, such as TMAH, and provides a more preferable etch pattern to that provided in the art.